In order to suppress the amount of carbon dioxide gas emitted from a vehicle, the weight of a vehicle body has been reduced by the use of a high-strength steel sheet. From the viewpoint of securing the safety of a passenger, a large number of high-strength steel sheets, in addition to a mild steel sheets, are used in a vehicle body. However, in order to further reduce the weight of a vehicle body, the strength of a high-strength steel sheet to be used is required to be higher than that of the related art.
However, generally, as the strength of a steel sheet is increased, the formability thereof is reduced. For example, Non-Patent Document 1 discloses that uniform elongation, which is important during drawing or stretch forming, deteriorates due to high-strengthening.
Therefore, in order to use a high-strength steel sheet in, for example, suspension components or components of a vehicle body for absorbing collision energy, it is important to improve local deformability such as local ductility which contributes to formability such as burring workability or bending workability.
To that end, Non-Patent Document 2 discloses a method of improving uniform elongation at the same strength by preparing a complex metallographic structure of a steel sheet.
Non-Patent Document 3 discloses a method of controlling a metallographic structure in which local deformability, represented by bendability, hole expansibility, or burring workability, is improved by inclusion control, single structuring, and a reduction in hardness difference between structures. In this method, a single structure is prepared by structure control to improve hole expansibility. In order to prepare a single structure, basically, a heat treatment from an austenitic single phase is required in this method as disclosed in Non-Patent Document 4.
In addition, Non-Patent Document 4 discloses a technique of increasing strength and securing ductility at the same time in which cooling after hot rolling is controlled to control a metallographic structure; and a precipitate and a transformation structure are controlled to obtain appropriate fractions of ferrite and bainite.
However, the above-described techniques are the methods of improving local deformability which depend on structure control, and greatly affect the structure formation of a base.
Meanwhile, techniques relating to the improvement of material properties by an increase in rolling reduction during continuous hot rolling are disclosed in the related art. These techniques are so-called grain refinement techniques. For example, Non-Patent Document 5 discloses a technique of increasing strength and toughness by grain refinement in which large reduction is performed in an austenite region in a lowest possible temperature range to transform non-recrystallized austenite into ferrite and thus to facilitate the grain refinement of ferrite which is the primary phase of a product. However, measures for improving local deformability that the invention is to solve is not disclosed at all.